Navigable aqueducts (sometimes called water bridges) are bridge structures which carry waterway canals over other rivers, valleys, railways or roads. They are primarily distinguished by their size, carrying a larger cross-section of water than most water-supply aqueducts. Although Roman aqueducts were sometimes used for transport, aqueducts were not generally used until the 17th century when the problems of summit level canals had been solved and modern canal systems started to appear. The 662-metre long steel Briare aqueduct carrying the Canal de Briare over the River Loire was built in 1896, and remained the longest navigable aqueduct in the world, until the 21st century when the Magdeburg Water Bridge in Germany took the title.
Early aqueducts such as the three on the Canal du Midi were stone or brick arches, the longest span being 18.3 metres on the Cesse aqueduct, built in 1690. However the weight of the construction to support the trough with the clay or other lining to make it waterproof made these structures clumsy and it was not until 1796 that the first large cast iron aqueduct was built at Longdon-on-Tern by Thomas Telford on the Shrewsbury Canal. It has a total length of 57 metres across three intermediate piers. Within ten years Telford had completed the far more ambitious Pontcysyllte Aqueduct on the Llangollen Canal over the River Dee valley—total length 307 metres. Other cast iron aqueducts followed such as the single span Stanley Ferry Aqueduct on the Calder and Hebble Navigation, in 1839 with its innovative 50-metre through arch design.
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